The present invention relates to hydrostatic transmission apparatus for a mobile machine having a front group of drive members and a rear group of drive members, a pump, a front motor and a rear motor contributing to driving respective ones of the front and rear groups of drive members; a first one of said motors referred to as the “first linking motor” and a second one of said motors referred to as the “first linked motor” and connected to said linking motor via a series duct also being connected to the main orifices of the pump, thereby constituting a closed transmission circuit; at least one pressure-limiting valve being interposed between said series duct and a low-pressure circuit.
Such a hydrostatic transmission configuration using a series connection arrangement offers the advantage of making it possible to synchronize the front and rear wheels of a vehicle on difficult terrain and to reduce the risk of wheel spin.
The term “upstream motor” is used below for a motor connected to a series duct and to the delivery orifice of the pump, and the term “downstream motor” is used below for a motor connected to a series duct and to the inlet orifice of the pump. It should also be noted that the terms “linking” and “linked” are independent of whether the motors they designate are upstream motors or downstream motors.
The pressure-limiting valve interposed between the series duct and the low-pressure circuit is for preventing excessive pressure from appearing in the series duct, in particular in certain specific situations that can give rise to such excessive pressure.
Such a problematic situation is constituted, for example, on certain difficult terrains, by violent and brief losses of grip of one or more wheels of the vehicle. Thus, if a wheel driven by an upstream motor suffers such a loss of grip (e.g. on going over a loose stone), that motor accelerates suddenly, thereby generating a rapid increase in the pressure in the series duct that feeds the downstream motor. Due to this sudden increase in pressure, the wheel driven by the downstream motor is destabilized because the torque transmitted by the downstream motor is increased suddenly instead of going gradually to its equilibrium value. The resulting pressure jolts can be very damaging for the hydraulic apparatus, and uncomfortable for the driver.
Another problematic situation is constituted by going round a turn. On turning, the wheels rotate at different speeds; this must be made possible despite the fact that the series duct normally constrains the front and rear motors interconnected by the series duct to rotate at the same speed, and without generating sudden changes in pressure in said series duct.
In order to avoid such sometimes-violent increases in pressure in said series duct, conventional apparatus uses a pressure-limiting valve set to the maximum value allowable by the circuit. That high pressure setting makes it possible to avoid untimely opening of the pressure-limiting valve, which might, in particular, give rise to insufficient pressure and thus to insufficient torque, precisely in situations (loss of grip on turning, etc.) for which high torque is necessary.
However, due to the high pressure setting of the pressure-limiting valve, the pressure can be relatively high in the series duct, and, in addition, sudden variations in pressure remain possible. All this gives rise to unnecessary fatigue in the structures.
Another known solution for solving the problems of pressure increases during turning consists in making provision for opening of the value that is used as a pressure-limiting valve to be dependent on the steering angle through which the wheels are steered, that valve removing the excess flow rate that can appear when the two wheels driven by the hydraulic motors linked via a series duct do not travel the same distance, which applies during turning. However, that solution does not cover all of the above-mentioned problematic situations, because, in particular, it does not eliminate the risks of a sudden increase in pressure in the series duct while the vehicle is traveling in a straight line.
An object of the present invention is, in apparatus of the above-mentioned type, to avoid or at least to limit such sudden increases in pressure, without however suffering from the above-mentioned drawbacks.
This object is achieved by the fact that said at least one pressure-limiting valve is suitable for connecting said series duct to the low-pressure circuit while being controlled as a function of the high pressure of the pump.
Thus, in accordance with the invention, the pressure-limiting valve can connect the series duct to the low-pressure duct when the pressure in said series duct reaches a value determined as a function of the high pressure of the pump. The high pressure of the pump should be understood herein as being the higher of the pressures measured at the main orifices of the pump. Said high pressure represents the load on the system: it corresponds precisely to the torque necessary to drive the vehicle at the precise moment in question.
By making the maximum allowable pressure in the series duct or “limit pressure” dependent upon said high pressure of the pump, the torque transferred instantaneously to the wheels driven by the motors fed in series is limited to a value that is mechanically acceptable. In addition, by means of the invention, the pressure in the series duct is adjusted as a function of the high pressure of the pump, i.e. as a function of the drive torque necessary at the instant in question for the vehicle. Thus, this pressure is adjusted in a manner such that the motors linked to the series duct are not unnecessarily deprived of drive torque, while also avoiding excessive increases in pressure that might give rise to loss of grip.
For example, if, at a given time, a pressure of 150 bars is necessary to cause the vehicle in question to advance, and if an upstream wheel loses grip, the torque that the corresponding downstream motor can develop depends upon the pressure in the series duct between the two motors, said pressure being a function of the 150 bars; while if, under other travel conditions (e.g. while climbing a slope), a pressure of 400 bars is necessary to cause the vehicle to advance, and so the limit pressure to which the pressure in the series duct is limited is different (higher) and takes account of the 400 bars, making it possible for the downstream motor to develop different (higher) torque. The pressure in the series duct is therefore no longer a fixed pressure corresponding to the pressure setting of the above-mentioned prior art fixed valve, which pressure could be 450 bars and could, in the same way as in the preceding situations, give rise to loss of grip of the wheel driven by the downstream motor.
In addition, controlling the pressure-limiting valve as a function of the high-pressure of the pump rather than as a function of its delivery pressure is particularly advantageous for coping successfully with the specific situation arising when the vehicle starts going down a steep slope. The high pressure generated by the weight of the vehicle then occurs in the return duct from the motors towards the pump (the motors then operate as pumps). The delivery pressure of the pump might thus become equal to the pressure of the low-pressure circuit. In this situation, while the pressures in the main ducts are inverted relative to a “normal” situation in which the motors operate as motors, it is necessary for the pressure in the series duct to be able to increase in order to maintain drive torque on the wheels driven by the motor at the front of the vehicle, which motor is preferably the upstream motor. This result is made possible in accordance with the invention by means of the fact that the pressure-limiting valve is controlled as a function of the high pressure of the pump.
In addition, advantageously, said at least one pressure-limiting valve is controlled on the basis of a fraction F of the high pressure of the pump. In particular, it is possible to choose to make provision for said valve to limit the pressure in the series duct to a limit pressure that is a fraction F of the high pressure of the pump, i.e. for said valve to connect the series duct to the low-pressure circuit when the pressure in said duct reaches said fraction F of the high pressure of the pump. This technical solution is both effective and simple to implement as shown by the embodiments presented below.
In general, said fraction F has a predetermined value. Advantageously, said fraction F is less than 1, by lying, for example, in the range 0.7 to 0.95. By choosing a fraction F that is less than 1, the pressure-limiting effect is increased, and the circuits are relieved. For particular applications, said fraction may lie in the range 0.4 to 0.7.
In this situation, the value of the pressure setting of the pressure-limiting valve is less than the high pressure of the pump, which pressure is very often (except in steep-slope situations, for example) the feed pressure of the upstream motor. There is therefore very often a positive pressure difference between the feed and the discharge of the upstream motor; said difference makes it possible for the upstream motor to be capable of delivering drive torque permanently, unlike what would occur if the pressure in the series duct could reach or even exceed the high pressure of the pump.
Limiting the fraction F to a value that is less than 1 is advantageous in particular in a turning situation and when the upstream motor drives the steerable wheels. The steerable wheels travel longer distances than the non-steerable wheels. Therefore, the upstream motor delivers a higher flow rate to the downstream motor than said downstream motor can absorb. The pressure in the series duct then increases, but remains less than the limit pressure in the series duct that is less than the high pressure of the pump. The difference between these two pressures is applied between the inlet orifice and the discharge orifice of the upstream motor, and said upstream motor therefore continues to deliver drive torque.
In addition, in certain particular situations, it is also possible to choose to make provision for said fraction to take a value greater than 1. This applies, for example, for a machine having 4 wheels and 4 motors with two cylinder capacities, or indeed 4 wheels and 2 motors having two cylinder capacities, and that is traveling in reverse. If a wheel equipped with motors having two cylinder capacities loses grip, a large rise in pressure (beyond the high pressure of the pump) can take place in the series line that goes to the other wheel on the same side. By choosing a fraction F having a sufficient value, e.g. 2 or indeed greater than 2, it is possible, in the event of spin or of loss of grip, to use this excessive pressure to give drive back to the vehicle, by giving drive torque back to the wheel linked via the series duct to the wheel suffering loss of grip.
Thus, in a first embodiment of the invention, the pressure-limiting valve may be controlled independently of the travel conditions of the vehicle, e.g. with a pressure-limiting valve controlled on the basis of a fraction F of the high pressure of the pump, and in particular of a predetermined fraction.
In other embodiments of the invention, it is also possible to make provision for said fraction to be adjustable, e.g. on fitting the valve, without this pressure taking account of the travel conditions. This makes it possible to simplify the on-board electronics.
However, it is advantageous for the control of the pressure-limiting valve to take account of particular circumstances.
Thus, advantageously, said at one pressure-limiting valve is also controlled as a function of at least one parameter relating to the operating conditions of the apparatus other than the high pressure of the pump. Said parameter may be at least one parameter chosen from among the inclination of the vehicle (traveling on the flat, climbing, or descending), the steering angle through which the wheels of the vehicle are steered, or the type of vehicle. It may, in particular, serve to determine said fraction F of the high pressure of the pump.
The use of such additional parameters for the control makes it possible in particular to have more power, or, conversely, to reduce the pressure to a greater extent in the series duct, when necessary, as a function of the circumstances as indicated by said parameters relating to the operating conditions.
For optimum use of the above-indicated parameters, the pressure-limiting valve may be controlled by an electronic control unit (ECU). Said unit may either control the pressure-limiting valve directly (the valve is then a solenoid valve), or else control it via an electrical-to-hydraulic conversion pilot stage (the pressure-limiting valve is then a hydraulically-controlled valve).
The hydrostatic transmission apparatus may also have certain features making it possible to optimize operation of said apparatus. First features aim to optimize the internal operation of the apparatus.
Thus, at least one constriction is interposed between said at least one pressure-limiting valve and the series duct(s) to which said pressure-limiting valve is connected. The presence of said constriction generates head loss and advantageously makes it possible to dimension said at least one pressure-limiting valve on the basis of pressures that are lower than the pressures of the series duct(s) to which said pressure-limiting valve is connected.
In addition, at least one non-return device is interposed between said at least one pressure-limiting valve and the series duct(s) to which said pressure-limiting valve is connected, so as to allow fluid to flow only from the series duct(s) towards said at least one pressure-limiting valve. This non-return device prevents the pressure-limiting valve from directing fluid towards the series duct, which is not desirable.
Advantageously, the hydrostatic transmission apparatus further includes control means making it possible to force the non-return device into the open or “through” position. In this position, the fluid can flow in both directions. Therefore, instead of only limiting the pressure in the series duct(s) to a limit pressure, it is possible to force the pressure in said duct(s) to establish itself at that value. This possibility is particularly advantageous in the following circumstances:                Firstly when spin is absolutely to be avoided on the front wheels (when drive is from the front with the front motors being the upstream motors). Under these conditions, it is necessary to limit the drive torque transmitted by the front motors or, in equivalent manner, to limit the pressure variation between the feed and the discharge of the motor(s) driving the front wheels.        
Advantageously, this result is obtained by forcing the non-return device to open. The pressure in the series ducts is thus forced to the limit value set at the outlet of the pressure-limiting valve. Since said limit value is usually close to the high pressure of the pump, the pressure difference between the feed and the discharge of the front motors is necessarily small, thereby making it possible to achieve the desired objective.                Secondly, when it is possible to guarantee minimum drive torque on the front wheels, in a situation when feeding is from the rear (the upstream motors are the rear motors), and in a turning situation. During turning, since the front wheels travel longer distances than the rear wheels, and in particular the inner wheels, the fluid flow-rate though the front motors can become insufficient, and the pressure variation between the feed and the discharge of each of said motors can become zero. The vehicle therefore becomes very difficult to handle.        
Advantageously, drive torque is given back to the front wheels, and the problem of poor handling is solved by forcing the non-return device into the open position. By doing so, a certain pressure difference is maintained between the feed and the exhaust of each of the front motors, which difference is equal to the difference between the high pressure of the pump and the limit pressure maintained by the pressure-limiting valve(s) in the series duct(s). Forcing the non-return valve thus delivers turning assistance.
It should be noted that the control means making it possible to force the non-return device to open may advantageously be triggered automatically by turning sensors, i.e. sensors that detect the inclinations of the wheels, e.g. inductive sensors. The control means of the non-return device may comprise a solenoid valve suitable for forcing the non-return device to open by applying a pilot pressure. Said pilot pressure may be the boost pressure, or preferably the high pressure of the pump.
In addition, as regards the pressure-limiting valve itself, various types of valve are suitable for performing the desired function.
Advantageously, said at least one pressure-limiting valve is a pressure limiter having a first port (G) suitable for being connected to the low-pressure circuit, a second port (U) suitable for being connected to a series duct, and a moving member suitable for being moved by control means between a first position in which it isolates the first and second ports and a second position in which it interconnects said ports. The control means receive the information about the value of the high pressure of the pump, and, on the basis of said information, cause the moving member of the valve to move between the above-mentioned two positions for reducing excessive pressure in the series duct.
In another embodiment, the above-mentioned pressure limiter may be a pressure regulator also provided with a third port (P) suitable for being connected to the duct of the high-pressure pump, the moving member being suitable, in its first position, for interconnecting the second and third ports. Compared with a conventional pressure limiter having only the two ports G and U, the presence of the port P makes it possible, in particular by interconnecting the second and third ports (P and U), to apply the limit pressure as a function of the control pressure in the port U connected to the series duct.
In addition, the pressure-limiting valve may be constituted by various means. In a preferred embodiment, said at least one pressure-limiting valve is a hydraulically-controlled valve, controlled by a pilot duct suitable for being connected to the orifice of the pump that is at the high pressure. A shuttle-valve connected to the two orifices of the main pump makes it possible to select the orifice of the pump that is at the high pressure, i.e. to select the higher pressure of the pump.
In another embodiment, the pressure-limiting valve may, for example, be an electrically-controlled valve.
Finally, other features for optimizing operation of the apparatus may be provided for adapting operation of said apparatus to match the stresses with which it must cope.
A first feature results from the observation that, when the vehicle is in a normal travel situation, e.g. on the road, the above-mentioned risks of excessive pressure to which the pressure-limiting valve responds are limited. For this reason, the apparatus may have two different configurations, one for the road and one for more difficult work conditions. Thus, advantageously, the apparatus may have a travel (or “road”) configuration in which said at least one pressure-limiting valve is suitable for connecting the series duct(s) to the low-pressure circuit while limiting the pressure in said duct(s) to the value of the feed pressure of the low-pressure circuit, and a work configuration in which said at least one pressure-limiting valve is suitable for connecting the series duct(s) to the low-pressure circuit while limiting the pressure in said duct(s) as a function of the high-pressure of the pump.
A second feature aims to make it possible to neutralize the limitation of the pressure in the series duct. For this purpose, advantageously, at least one solenoid valve is interposed between said at least one pressure-limiting valve and the series duct(s) to which said pressure-limiting valve is connected, said solenoid valve having a first position in which it connects the at least one pressure-limiting valve to said series duct(s), and a second position in which it isolates the at least one pressure-limiting valve from said series duct(s).
It can be necessary to make it possible to neutralize the pressure-limiting valve in order to maintain drive torque on the downstream motor, under certain particular circumstances. For example, if a wheel driven by an upstream motor is suffering a major loss of grip, said upstream motor tends to rotate faster and to absorb the entire flow-rate of the pump. The corresponding downstream motor thus receives an increased flow-rate, and the pressure in the series duct should therefore rise. However, if the pressure-limiting valve is active, the pressure in the series duct is limited by said valve, which is controlled as a function of the high pressure of the pump that has itself decreased under the effect of the above-mentioned over-consumption. Under these conditions, the feed pressure of the downstream motor can be insufficient; it is then desirable to neutralize the pressure-limiting valve temporarily, in order to enable the pressure to increase in the series duct so as to give back to the downstream motor the possibility of delivering sufficient drive torque. Conversely, it is recommended not to neutralize the pressure-limiting valve during turning.
Due to the variety of the situations that might require neutralization of the pressure-limiting valve, said solenoid valve may be actuated either by action from the driver of the vehicle using a suitable control (e.g. a push button or any other device), and/or automatically by means of a sensor, e.g. a proximity detector, giving straight-line or turning information to an electronic unit that can thus transmit to the solenoid valve a blocking signal in a straight-line situation or an opening signal in a turning situation.
It should also be noted that the above-mentioned additional features (work configuration and road configuration, neutralization of the pressure-limiting valve) can be brought together in a single solenoid valve, interposed between the pressure-limiting valve and the series duct(s).
For this purpose, a solenoid valve is used that is connected at its inlet to the outlet orifice of the pressure-limiting valve, and to the low-pressure circuit, and that is connected at its outlet to the series duct(s).
This solenoid valve has the same positions as the above-indicated positions of the solenoid valve for neutralizing the pressure-limiting, so as to connect the pressure-limiting valve to the series duct(s) or so as to isolate them. It also has a new third position, in which the low-pressure circuit is connected to the series duct(s).
The advantage of this solenoid valve is firstly to simplify the hydraulic transmission apparatus by combining various functions in a single part. But it is also, by means of said third position, to make it possible, in simple manner, for the apparatus to operate in the above-mentioned road configuration.
In general, the hydrostatic transmission apparatus of the present invention may equip any vehicle having the characteristics presented in the introduction. However, it should be noted that the apparatus is not limited to a vehicle having a single linking motor and a single linked motor: on the contrary, other motors, be they linking or linked, can also be part of the apparatus. In particular, the front group of motors and/or the rear group of motors may include dual motors, i.e. motors each made up of two sub-motors having distinct feed and/or discharge orifices.
For example, in another possible configuration of apparatus that is referred to as a “configuration with a dual motor”, said first linking motor is a first elementary motor of a first dual motor, and said first dual motor also has a second elementary motor that is fed by the pump in parallel with said first linking motor and with said first linked motor that are interconnected in series.
In another possible configuration, in addition to the configuration with a dual motor, the apparatus also includes a second dual motor, the first elementary motor of which is connected firstly via a second series duct to the first linked motor, and secondly to the pump, and of which the second elementary motor is fed by the pump in parallel with said first linking motor and with said first linked motor that are interconnected in series.
In yet another possible configuration, in addition to the configuration with a dual motor, the apparatus further includes another motor referred to as the “second linked motor”, and a second dual motor of which a first elementary motor is connected firstly via a second series duct to the second linked motor, and secondly to the pump, and of which the second elementary motor is fed by the pump in parallel with said first linking motor and with said first linked motor that are interconnected in series.
When the apparatus includes a plurality of series ducts, the apparatus may advantageously have only a single pressure-limiting valve interposed between the low-pressure circuit and one or more series ducts.